The present invention relates to an electromagnetically driven device and, more particularly, to an improvement of the device.
The present inventors have proposed, in the U.S. patent application Ser. No. 950,518, titled "ELECTROMAGNETIC DEVICE WITH DUST TIGHT ENCLOSURE" and filed on Oct. 11, 1978, an electromagnetic driven device which is diagrammatically shown in FIG. 1. In FIG. 1, a plunger 2 is slidingly inserted into a coil 1 for constituting a solenoid unit. The plunger 2 is urged towards an ejected position by a spring 6, but is normally held in an inserted position as shown by a magnetic attractive force between the plunger 2 and a permanent magnet 4 provided above or on one end of the coil 1. The coil 1 is connected to an electric power supply source including a battery E and a capacitor C which are connected in parallel to the coil 1. During the plunger 2 being held in the inserted position and when the coil 1 is energized, the attractive force acting from the permanent magnet 4 on the plunger 2 is counterbalanced with the magnetic force generated by the coil 1 to such an extent that the attractive force between the permanent magnet 4 and the plunger 2 is reduced or eliminated, thereby permitting the plunger 2 to move towards the ejected position by the biasing force of the spring 6.
In the device shown in FIG. 1, the employment of the permanent magnet 4 in combination with the solenoid unit has such an advantage that the device does not need to employ a mechanical supporting means for supporting the plunger 2 in the inserted position. Thus, the device shown in FIG. 1 is much simpler in construction than the conventional electromagnetically driven devices such as disclosed in U.S. Pat. No. 4,164,721 to Ishida et al., issued Aug. 14, 1979; Japanese Patent Laid-Open Publication No. 129627/1978, published on Nov. 11, 1978; and Japanese Patent Laid-Open Publication No. 619/1979, published on Jan. 6, 1979. All of the electromagnetically driven devices disclosed in these three references employ an electromagnet of a horse-shoe configuration.
However, the device shown in FIG. 1 has such a disadvantage that the coil 1 requires a considerably large amount of electric current for generating the counterbalancing magnetic force. In order to enable the device of FIG. 1 to be operable with a limited amount of current the capacitor C having a large capacitance is needed, resulting in a large size of power supplying circuit and a long time interval between two subsequent operations. Such a disadvantage is particularly serious when the device is to be employed in a small size apparatus with a quick sequential operation, such as a camera equipped with a motor drive unit.